Compositionally graded and porosity graded coatings using a solution precursor plasma spray process

ABSTRACT

A solution precursor plasma spray process (SPPS) produces compositionally graded and porosity graded coatings with no discrete interfaces and with reduced coating stress and improved coating durability. The disclosed SPPS process may fabricate superior thick monolithic thermal barrier coatings with engineered microstructural defects (cracks, pores) and thin, dense coatings for a variety of engineering applications. The disclosed coatings may possess a continuous variation of one or more physical properties selected from density, thermal conductivity, emissivity, thermal expansion coefficient, abrasion resistance, erosion resistance, hardness, fracture toughness, gas penetration resistance, liquid penetration resistance, elastic modulus, and strain tolerance. The disclosed coatings may possess a continuous variation of one or more chemical properties selected from corrosion resistance to solids, liquids, and/or gases, reactivity with liquids and prevent further penetration of liquids, reactivity with gases and prevent further penetration of gases, and varying diffusion coefficients for one or more chemical species.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 62/651,161, filed Mar. 31, 2018, which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention describes coatings, and methods of producing such coatings, that have one or more continuously graded features across the thickness of the coating.

The continuously graded feature may comprise, without limitation, one or more of chemical composition, phase composition, secondary phase distribution, secondary phase grain size, secondary phase volume fraction, volume of porosity, shape of pores, orientation of pores, distribution of pores, number of cracks, orientation of cracks, shape of cracks or width of cracks.

BACKGROUND OF THE INVENTION

Until 10 years ago, most coatings were monolithic in structure and composition. The demands for increased coating performance have led to the development of multi-layer coatings, in which each layer serves a particular engineering function. These multi-layer coatings all have discrete interfaces that are sources of increased localized stress and reduced service lives.

Discrete interfaces, produced by either compositional or porosity level, differences, lead to higher localized stresses at interfaces [1], particularly theoretically infinite interlaminar shear stresses at component free edges. For elastic behavior, the infinite stresses at edges occur if the two layers have different effective elastic properties as captured by the Dundurs parameter. Each of the two non-dimensional Dundurs parameters are functions the elastic constants of the materials on both sides of the interface. For example, the alpha parameter can be expressed simply as the difference of the two Youngs moduli over the sum of them. Effective elastic properties differences will occur if there is a composition difference between layers or if there is a porosity difference between layers. Dundurs parameters are illustrated in FIG. 1, taken from reference [3] below. FIG. 1 illustrates and defines a bi-material consisting of layers A and B, the local geometry, and the Dundurs parameters.

Discrete layers also can cause adverse stress differences if there is a differential shrinkage rate at temperature due to sintering, which is expected for layers with different compositions or layers with different amounts and/or types of porosity. High interface stresses also result if the adjacent layers have different coefficients of expansion, as is expected for layers of different compositions but not of different porosity. For both situations, grading the interface disperses the stress from differential shrinkage over a larger less concentrated area, thus improving durability. Finally, in a compositionally layered coating, there is a discrete interface, which usually has inferior bond strength. The mechanics are such that the highest stresses (infinite at the free edge) are concentrated at that interface. The co-location of less than ideal bonding at the highest stress location significantly reduces coating durability. Compositional grading spreads the less than ideal bonding over a volume and that will not be co-located with the highly concentrated interlaminar shear stress. The grading also eliminates the infinite stress at the free edge. It is well recognized that continuous grading of composition and porosity content is desirable to reduce stresses and increase coating durability. One example among many is provided in reference [2], where reduced stresses were calculated for a 5-layer coating designed to approximate a graded coating. In addition, at elevated temperatures, the nearly infinite compositional gradients across discrete interfaces in a coating made of two different materials leads to accelerated interdiffusion and loss of coating properties. Accordingly, graded porosity and graded compositions are well recognized as desirable, but have been difficult to produce economically with current production coating processes.

There are many production processes for making engineered coatings for use as thermal barriers, corrosion resistance, wear resistance, oxygen-transport membranes, etc. These include air plasma spray (APS), election beam physical vapor deposition, cathodic arc. All of these processes can be used to make multi-layered coatings. In principle, the APS process using powders or suspensions could make a type of graded coating in which a structure of finite sized regions of materials A and Materials B could be deposited. However in the presently disclosed invention, the mixing of constituents occurs in a chemical solution and the mixing is at the molecular level. The resulting structure is fundamentally different because it is not made from finite domains of A and B dictated by the feed materials but is molecularly mixed and the structure dictated by the phase diagram or its equivalent when applied to the rapid heating of the solution precursor spray process.

REFERENCES

-   1. E. Martin, D. Leguillon, N. Carrere, “A two fold strength and     toughness criterion for the onset of free-edge shear delamination in     angle-ply laminates,” International Journal of Solids and     Structures, Vol. 47, issue 9, 2010, pp. 1297-1305. -   2. K. A. Khor, Z. L. Dong and Y. W. Gu, “Plasma sprayed functionally     grade thermal barrier coatings,” Materials Letters, Vol. 38, issue     6, 1999, pp. 437-444. -   3. R. Desmorat and F. A. Leckie, “Singularities in Bimaterials: a     parametric study of an isotropic/anisotropic joint,” European     Journal of Mechanics, Vol. 17, No. 1, 1998, pp. 33-52.

SUMMARY OF THE INVENTION

The disclosed invention describes new capabilities of the solution precursor plasma spray process (SPPS) to make compositionally graded and porosity graded coatings with no discrete interfaces and with reduced coating stress and improved coating durability. The SPPS process may fabricate superior thick monolithic thermal barrier coatings with engineered microstructural defects (cracks, pores) and thin, dense coatings for a variety of engineering applications.

Some embodiments of this invention comprise coatings with one or more continuously graded features as described above, produced through a solution precursor plasma spray process. Some embodiments of this invention result in the continuous variation of one or more features as described above resulting in continuous variation of one or more physical or chemical properties of the coating. The physical properties so varied may include, without limitation, density, thermal conductivity, emissivity, thermal expansion coefficient, abrasion resistance, erosion resistance, hardness, fracture toughness, gas penetration resistance, liquid penetration resistance, elastic modulus, or strain tolerance. Chemical properties may include, without limitation, corrosion resistance to solids (e.g. dust, calcium magnesium aluminosilicates-CMAS), corrosion resistance to liquids (e.g. molten salts, molten CMAS, water, hydrocarbons, organic liquids, acids, alkalis, biofluids), corrosion resistance to gases (e.g. oxygen, hydrogen, water vapor, hydrocarbons, volatile organic compounds, CO₂, acid fumes), ability to react with liquids and prevent further penetration of liquids (e.g. CMAS), ability to react with gases and prevent further penetration of gases (e.g. oxygen), and or varying diffusion coefficients for one or more chemical species.

Some embodiments of said graded coating provide improved performance that may include, without limitation, one or more of greater reliability, longer lifetime, improved thermal cycling resistance, improved thermal shock resistance, improved corrosion protection, and/or improved functionality for the coatings on metallic, intermetallic or ceramic substrates on which these coatings are applied.

In some embodiments of the presently disclosed invention, one or more continuously varying features are produced in the coating by continuously varying one or more processing parameters in the solution precursor plasma spray process. In some embodiments of the present invention, the process parameters that may be continuously varied so may include, without limitation, the injection rates of one or more solution precursors being injected into the plasma flame or torch, gun power, raster scan rate, stand-off distance, injection rate of secondary additives as fuels (e.g. urea) that can change the flame temperature, and/or gas flow rates.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 illustrates and defines a bi-material consisting of layers A and B, the local geometry, and the Dundurs parameters;

FIG. 2 is a schematic representation of the Solution Precursor Plasma Spray (SPPS) system and process within the scope of the disclosed invention;

FIGS. 3A-3C are schematic representations of flow control provided by a back-pressure regulator;

FIG. 4. Schematic showing SPPS TBC with graded porosity, consisting of a dense inner layer for enhanced thermal cycle durability and a porous outer layer for reduced thermal conductivity;

FIGS. 5A-5C show graded porosity used for two specific gas turbine applications;

FIG. 6 shows SEM micrographs of the microstructures of SPPS-produced yttria-stabilized zirconia (YSZ) TBCs with varying levels of porosity.

FIG. 7 shows SEM micrographs of the microstructures of planar arrays of porosity in SPPS YSZ TBCs varied over wide strengths/intensities; and

FIG. 8 shows SEM micrographs of the microstructures of planar arrays of porosity, IPBs, of varying strength in SPPS YAG TBCs.

DESCRIPTION OF THE INVENTION

The SPPS process, which will now be described, does have the processing flexibility, to make coating with graded porosity and composition. Moreover, this can be accomplished “on the fly” by a simple variation in a single process control. SST was formed to develop and commercialize the SPPS process for making novel and improved coatings. The SPPS process shown in FIG. 2 is a modification of the widely used, commercial air plasma spray (APS) process. In the APS process, powder is injected into the plasma plume, melted, and the molten particles are deposited as a series of splats (pancakes) to form successive layers of the coating. In the SPPS process, a liquid precursor, containing the cations to be deposited, is injected into the plasma plume. The atomized precursor droplets undergo a series of physical and chemical changes in the plasma and are deposited as ultra-fine splats, whose area is two orders-of-magnitude smaller than the splats from the APS process. This basic process can also be implemented with other thermal torches including, high velocity oxygen fuel torches, high velocity air fuel torches, flame spray, detonation spray, and Uniform Melt State processing.

Methods for Making Compositionally Graded Coatings

The SPPS coating can be compositionally graded by continuously altering the precursor chemical composition. To illustrate this, we will deal with a system for just two compositions A and B. For more complex systems involving A, B, C . . . , one can use a multi-channel version of the following two plans.

In both methods, it is necessary to provide metered flow of the two liquids A and B. Metered flow may be achieved by a number of methods, including, but not limited to: electrically controlled peristaltic pumps, variable rate syringe pumps with switching valves that allow continuous operation, electrically controlled servo valves and electrically controlled valves controlling back pressure on a metering flow orifice with the forward flow side held at a constant pressure by a regulated gas tank overpressure. The backpressure regulation approach is novel and is shown below in FIGS. 3A-3C. FIG. 3A shows a top view of component parts of the system to meter flow rate using back pressure regulation. FIG. 3B is a side view of the back pressure regulation system. FIG. 3C is another side view of the back pressure regulation system with a non-limiting description of the spatial dimensions of the system. The flow rate through a restrictive orifice is dependent on the pressure difference between the inlet and outlet side of the orifice. The apparatus uses constant pressure on the inlet side and electrically controlled backpressure that dictates flow. This also makes the system insensitive to changes in pressure downstream. By these and similar means, the desired flow vs time from precursor sources A and B or more complex precursor sources A, B, C, D, etc. can be controlled. Finally, all of the above methods can benefit if greater precision of control is needed by separately measuring the flow of A and B and potentially using feedback from this measurement to establish a higher level of individual flow control. Such flow measurements can be achieved by any appropriate means, but specifically should include, paddle wheel, Coriolis meters, and continuously weighing the reservoir of the precursor(s) for which the flow rate is to be controlled.

Various methods of mixing metered flows precursor source liquids may be used. The following are two non-limiting examples of mixing precursor source liquids.

Method 1. The controlled or controlled and metered source of A and B can be directed into a mixing tank, in such a way that the composition of the mix in the tank changes in a smooth and continuous way, as required to make graded coatings. This flow with continuously changing chemistry over time is then fed to the solution thermal spray process to make a continuously graded coating.

Method 2. The controlled or controlled and metered source of flow of A and B can be fed to an engineered fluid mixer, typically a turbulent mixer. This then produces a continuous stream with smoothly varying chemical composition. This method allows mixing to occur close to the injection location because such mixers are small and there is no need for a larger mixing tank. As a result, this method will be capable of more rapid changes in composition. Small mixers of the type mentioned are commercially available. For more than A and B, one can use a series of binary mixers such, for example, mixer 1 mixes A and B and AB is then mixed with C in another mixer, and so on to as many components as desired.

Multi-layer coatings have become more widely adapted as durability and performance demands on coatings have steadily increased. Each component of the multi-layer coating serves a specific purpose. These coatings suffer from increased cost because they require separate feed stocks and increased processing times. They also suffer from poorer durability because of the higher stresses at layer interfaces. The SPPS process has the capability of making these same multi-layer coatings at lower cost and increased durability. A number of examples are cited:

Thermal barrier coating (TBC) used to protect metals from high temperatures most typically but not limited to gas turbines but can also be used in internal combustion engines, high speed flight vehicles, heat exchangers, and other applications involving high temperature exposure. The TBC may, for example, have a high temperature-resistant composition at the surface and low thermal conductivity layer underneath.

TBC with corrosion resistance layer at surface and low thermal conductivity layer underneath.

TBC with erosion resistant layer at surface and low thermal conductivity underneath

TBC with abradable layer at surface and low thermal conductivity underneath.

Combinations of the above requiring 3 or more layers.

Methods for Making Porosity Graded Coatings.

There is direct experimental evidence that coating porosity varies with the parameters used to create the coating. At least three specific parameters that greatly affect coating porosity are:

1. Composition of the precursor, including additives and solvents.

2. Injection feed rate of the precursor.

3. Liquid injection orifice size of for atomizing injection.

For atomizing injectors the atomizing gas pressure may range from 5 to 40 psi, preferably 26-30 psi. Injector orifice size for stream injection may range from 0.005 inches to 0.025 inches, preferably 0.008 to 0.0016 inches

For Method 1, the same methods as described in compositional grading can be used to make porosity graded coatings. However, in this case, one can control the mixing of additives, such as urea or polyvinyl alcohol (PVA), that do not become part of the final coating composition, but influence porosity or control the solute to solvent ratio or the solvent chemistry. Specific examples could include, but are not limited to varying the amount of dissolved urea, or PVA. Or for example adding an organic solvent like ethanol to a water-based solution. Many other combinations are possible to vary porosity by varying the precursor composition with additives and solvents.

For Method 2, using the methods described above for flow control, a graded porosity coating can be produced by continuously varying the total precursor flow rate.

FIG. 4 shows schematically continuous grading of uniformly distributed porosity in which low porosity near the bond coat provides improved thermal cycle durability and progressively higher porosity toward the surface provides lower thermal conductivity, and perhaps increased abradability.

FIGS. 5A-5C show graded porosity used for two specific gas turbine applications. Current air plasma spray (APS) TBCs use functionally graded coating that require process interruptions and produce discrete, high stress interfaces (FIG. 5A). The SPPS process will produce graded porosity TBCs “on the fly” with no discrete interfaces. FIGS. 5B and 5C show grading applicable to combustor and turbine outer air seals, respectfully. The graded multi-layer coating has a dense inner layer next to the bond coated superalloy for improved bond strength, then graded to a highly porous mid-coating for reduced thermal conductivity. For the combustor application there is grading to a denser surface layer for improved erosion resistance. For the turbine outer air seal, whose surface has to be abradable, there is a more porous surface layer.

FIG. 5A shows that a current TBC with varying porosity has a discrete interface. FIG. 5B shows continuously graded porosity for a combustor application going from dense at the bond coat to porous through most of the coating and dense again at the outer surface. FIG. 5C shows continuously graded porosity of a turbine outer air seal application going from a dense inner layer to porous through most of the coating and a highly porous surface layer.

FIG. 6 shows SEM micrographs of varying levels of porosity, from 15 to 37%, in SPPS YSZ TBCs that were produced by varying the precursor injection feed rate. All the other process parameters were held constant and are described in Table 1. Grading the porosity in SPPS coatings in this manner can be performed continuously with changes in the precursor feed rate “on the fly,” using the control techniques described previously.

TABLE 1 Process parameters used to produce the SPPS YSZ TBCs samples shown in FIG. 6. Parameters Details Precursor YSZ acetate solution Precursor Injection Stream injection Raster Scan Step 0.5-5 mm Feed Rate 23-106 mL/min 9 MB Gun Power 35-45 kW Ar Flow 120-140 SCFH H₂ Flow 14-18 SCFH Standoff Distance 40-50 mm

FIG. 7 shows SEM micrographs of varying levels of porosity produced by varying intensities of planar porosity, called inter-pass boundaries (IPBs). Since IPBs are aligned normal to the direction of heat flow in a TBC coated component, they are highly efficient in reducing thermal conductivity. The greater the intensity/strength of IPBs, the greater is the reduction in thermal conductivity. FIG. 7 shows that IPB strength was varied from light to medium to heavy by changes in the plasma gun raster scan step height. The raster scan step height changes can be made in-process and on the fly by programming the robot holding the component. All the other process parameters were held constant and are described in Table 2.

TABLE 2 Process parameters used to produce the SPPS YSZ TBCs samples shown in FIG. 7. Parameters Details Precursor YSZ acetate solution Precursor Injection Stream injection Raster Scan Step 2-6 mm Feed Rate 38 mL/min 9 MB Gun Power 35-49 kW Ar Flow 120-140 SCFH H₂ Flow 14-18 SCFH Standoff Distance 40-45 mm

FIG. 8 shows SEM micrographs of planar arrays of porosity and IPBs of varying strength in SPPS YAG TBCs. All the other process parameters were held constant and are described in Table 2.

TABLE 3 Process parameters used to produce the SPPS YSZ TBCs samples shown in FIG. 8. Parameters Details Gun 9 MB Precursor YAG nitrate-nitrate Precursor Injection Atomizing injection, 10-20 psi Raster Scan Step 1-2 mm Feed Rate 18-38 mL/min Gun Power 40-50 kW Ar Flow 100-130 SCFH H₂ Flow 8-14 SCFH Standoff Distance 35-40 mm

In addition to air plasma spray, the continuously changing precursors for compositional gradient and injection flow rate for porosity can be deposited in gas flame jets, liquid fuel jets, high velocity oxygen fuel jets, and detonation gun jets.

From the foregoing, it will be appreciated that compositionally graded and porosity graded coatings may be prepared using a solution precursor plasma spray process. The disclosed coatings have one or more continuously graded features across the thickness of the coating. Methods of producing such coatings involve continuously varying one or more parameters or compositional precursors of the solution precursor plasma spray process. Non-limiting examples of interrelationships between the continuously graded features and the continuously varied process parameters are set forth in Table 4, below.

Impact of Feature Parameters Parameter Impact of Feature Volume fraction of Flow rate of Increasing flow rate Thermal porosity precursor increases porosity conductivity, Elastic Composition of Controls gas release modulus, gas/fluid precursor and gas trapping permeability, stain tolerance Size and size Precursor viscosity Increasing viscosity Thermal distribution of increases pore size conductivity, Elastic porosity Injector type and Stream injection may modulus, gas/fluid location of injector result in larger pores permeability, stain relative to thermal jet tolerance Flow rate of Increasing flow rate precursor results in larger porosity Raster scan step size Smaller raster scan (spatial offset step size means more between torch prominent inter pass passes) boundaries (flat, linearly oriented porosity) Chemical Ratio of flow rates of Increasing the Basically everything composition different precursor relative flow rate of a ingredients used particular precursor results in greater content of that species in the final coating Secondary phase Ratio of flow rates of Increasing the Basically everything composition different precursor relative flow rate of a ingredients used particular precursor results in greater content of that species in the final coating Gas velocity Greater gas velocity can result in greater non equilibrium phases Injector type Injectors that can produce finer droplets generally lower cooling rates and less metastable phases Secondary phase Ratio of flow rates of Increasing the size and distribution different precursor relative flow rate of a ingredients used particular precursor results in greater content of that species in the final coating Gas velocity Greater gas velocity can result in greater non equilibrium phases Injector type Injectors that can Substrate produce finer temperature droplets generally lower cooling rates and less metastable phases Crack density and Increased deposition Promoted by less Strain tolerance, fluid orientation of unpyrolized perfect entrainment penetration, CMAS material produces of the liquid in the resistance greater cracking by plasma jet, Achieved pyrolization by having either too shrinkage low liquid jet Finer arriving momentum or too particles that are high compared to deflected by injection location stagnation layer Adjusted by flow (Stokes number rate, injector effect) diameter and injection location Width of cracks Factors same as for Strain tolerance, fluid crack density. penetration, CMAS resistance

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

1. A coating deposited by a solution precursor plasma spray process (SPPS) that is continuously graded throughout a cross-section of the coating as to composition, porosity, or composition and porosity.
 2. The coating of claim 1 produced by low and high enthalpy thermal spray guns, selected from an air plasma spray, a high velocity oxygen fuel spray, a flame spray, and a detonation gun.
 3. The coating of claim 1 in which some or all sub-layers are homogeneous as to composition or porosity and for which grading is produced between sub-layers, such that there are no discrete interfaces between sub-layers.
 4. The coating of claim 1 that is devoid of discrete interfaces based on composition and porosity.
 5. The coating of claim 1 where the coating possesses a continuous variation of one or more physical properties throughout a cross-section of the coating, wherein the one or more physical properties are selected from density, thermal conductivity, emissivity, thermal expansion coefficient, abrasion resistance, erosion resistance, hardness, fracture toughness, gas penetration resistance, liquid penetration resistance, elastic modulus, and strain tolerance.
 6. The coating of claim 1 where the coating possesses a continuous variation of one or more chemical properties selected from corrosion resistance to solids, corrosion resistance to liquids, corrosion resistance to gases, reactivity with liquids and prevent further penetration of liquids, reactivity with gases and prevent further penetration of gases, and varying diffusion coefficients for one or more chemical species.
 7. A process for preparing a compositionally graded or porosity graded coating using a solution precursor plasma spray process comprising continuously varying one or more process parameters in the solution precursor plasma spray process, wherein the process parameters are selected from an injection rate of one or more solution precursors being injected into a plasma flame, plasma gun power, plasma gun raster scan rate, plasma gun stand-off distance, an injection rate of a precursor of one or more secondary additives to the solution precursors that can change the plasma flame temperature, and torch gas flow rates, wherein the process produces a coating that is continuously graded as to composition or porosity through a cross-section of the coating.
 8. The process of claim 7 wherein a plurality of solution precursors of different composition are injected using two or more injectors and wherein a relative proportion of the plurality of solution precursors is varied continuously during deposition of part or all of the coating.
 9. The process of claim 7 in which sub-layers are homogeneous as to composition or porosity and for which grading is produced between sub-layers, such that there are no discrete interfaces between sub-layers.
 10. The process of claim 7 wherein multiple injectors are used to produce the coating which is compositionally graded over part or all of the coating thickness.
 11. The process of claim 7 wherein a plurality of solution precursors are mixed at or near the plasma torch, and wherein relative proportions of the plurality of solution precursors are continuously varied during all or part of the solution precursor plasma spray process to produce a compositionally graded coating.
 12. The process of claim 7 wherein flow rate and mixing of solution precursors are controlled and varied by a peristaltic pump, a syringe pump with or without switching valves that allow continuous operation, a gear pump, a reciprocating pump, a turbine pump, and a metering orifice and back pressure regulator with or without feedback control from a flow meter.
 13. The process of claim 12, wherein the flow rate and mixing of solution precursors are controlled and varied by a metering orifice and back pressure regulator with feedback control from a Coriolis meter. 